Aaron Carter was found dead in his bathtub, but his family now says he didn't die from drowning ... instead, they believe drugs killed him.
Aaron's fiancée, Melanie Martin, and mother, Jane Carter, tell TMZ ... the L.A. County Coroner's Office told them there was no water found in Aaron's lungs, which rules out drowning as a potential cause of death.
Melanie and Jane say the coroner is still waiting on toxicology results to determine what was in Aaron's system when he died, and the family wants law enforcement to investigate an alleged drug deal the night of Aaron's death.
TMZ broke the story, cops were called to Aaron's L.A. area home back in November for a report of a drowning.
Melanie has Aaron's phone and she says she found text messages with an unidentified person telling Aaron he owed $800 for an unknown substance. She says the conversation includes Aaron telling the person he didn't need the substance anymore, to which the person responded that it didn't matter and he still owed money.